1. Field of the Invention
The present invention relates to an image reader for optically and electronically sensing and reading an image, which is recorded on a suitable recording medium, such as a transparency, a sheet of paper or the like, by illuminating the recording medium with a light source and by scanning the illuminated recording medium with a line image sensor.
2. Description of the Related Art
Such an image reader per se is well known, and is used in peripheral equipment associated with an image-processing computer for retrieving an image. The image reader includes a solid-state line image sensor, such as a CCD (charge-coupled device) image sensor, a suitable light source for illuminating a recording medium carrying an image recorded thereon, and an optical system for focusing light rays, passing through or reflected by the recording medium, on a light receiving surface of the CCD line image sensor.
The CCD line image sensor includes a plurality of CCD elements aligned with each other, and each of the CCD elements generates and accumulates an electric charge in accordance with a received amount of light rays. As is well known, the CCD line image sensor possesses an electronic shutter function, and a time of electric-charge-accumulation or a time of exposure may be suitably regulated by using the electronic shutter function. As long as the CCD line image sensor is exposed to the light rays (if the electronic shutter is not "closed" and exposure continues), a degree of electric charge in each of the CCD elements is gradually increased, and the CCD elements finally reach saturation with the accumulated electric charges.
In operation, the recording medium, while being illuminated with light rays emitted from the light source, is scanned with the CCD line image sensor. The CCD line image sensor is exposed to the light rays, passing through or reflected by the recording medium, whereby a recorded image of the recording medium is optically and electronically sensed and read. During the reading of the recorded image from the recording medium, the time of exposure, over a period of which the CCD line image sensor is exposed to the light rays, should be optimally regulated before the read image can be obtained with the best contrast.
An optimal exposure time is varied in accordance with a change in transparency of a recorded image due to the reading of another recording medium. Accordingly, the optimal exposure time must be determined in accordance with the transparency of the recording medium. Conventionally, prior to a regular scanning operation for sensing and reading the recorded image from the recording medium, a pre-scanning operation is performed in order to determine an optimal exposure time with respect to the recorded image of the recording medium concerned.
Nevertheless, conventionally, it is impossible to accurately determine the optimum exposure time, because a method for determining the optimum exposure time is based on an inaccurate assumption that there is a directly linear relationship between a time of exposure and a degree of electric charge accumulation in the CCD line sensor, as discussed hereinafter in detail.